1. Field of the Invention
The present invention relates to a pixel structure for a liquid crystal display and more particularly to a high speed response pixel structure for a liquid crystal display.
2. Description of the Related Art
Twisted nematic (TN) cells, which at present are widely used in TFT color liquid crystal display devices (TFT/LCDs), have a small view-field angle. This results in a decrease in contrast and image inversion when an LCD panel surface is viewed from an oblique direction. Various methods have been proposed to solve this problem, i.e., to realize a wide view-field angle. Among these methods is an orientation division method in which each pixel of an LCD is divided into two parts and orientation is affected in different directions in the two parts.
However, these methods require cumbersome manufacturing steps. For example, in the case of the orientation division method, two rubbing steps are required. These steps include the further steps of coating, baking, patterning, developing and removing photoresist.
In recent years, studies on an OCB cell that is to be used as a liquid crystal cell instead of a TN cell have been made. If the OCB cell technique is used, it becomes possible to obtain a wide view-field angle more easily than with the orientation division method as well as a high-speed response characteristic that is one-order faster than with conventional TN cells.
FIG. 1 is a perspective view illustrating the structure of an OCB cell. A liquid crystal material that exhibits splay orientation 104 is sealed between two (top and bottom) glass substrates 100 and 102. Polarizing plates 106 and 108 are disposed outside the two respective glass substrates 100 and 102. When a voltage is applied to the glass substrates 100 and 102, the liquid crystal material is transformed from splay orientation 104 to bend orientation 110 as shown in FIG. 1B. In a bend orientation 110 cell, since top and bottom liquid crystal molecules are always oriented symmetrically, the view-field-angle dependence is symmetrical around the AA′ line. An optically compensated bend (OCB) mode LCDs compensates for the birefringence of liquid crystal molecules so as to obtain the uniform viewing angle characteristic at all directions.
An OCB cell is in a splay orientation state when no bias voltage is applied thereto, and exhibits a bend orientation state when a given high voltage is applied thereto. To allow an OCB cell to operate as a liquid crystal display device, the cell must be transformed from a splay orientation to bend orientation at the start of operation. This process requires a restart time, which reduces the response speed.
FIG. 2A shows a pixel structure plan diagram of a thin-film transistor LCD. The gate electrode 306a of the switch transistor 306 is connected to the scan line 302. The drain electrode 306b of the switch transistor 306 is connected to the pixel electrode 308 and the source electrode 306c is connected to the video data line 304. A common line 310 is used as the common electrode of the pixel electrode 308. The switch transistor 306 is usually a thin-film transistor (TFT) that is deposited on a transparent substrate such as glass. By scanning the scan lines 302 and in accordance with the scan signals, all of the switch transistors 306 in a given scan line 302 are turned on. At the same time, video signals are provided in the video data lines synchronously with the selected scan line 302.
FIG. 2B is a cross-sectional view along the BB′ line in the FIG. 2A. A liquid crystal material 326 is sealed between two (top and bottom) glass substrates 320 and 322. A conductor electrode 324 is located on the top glass substrate 320. Referring to FIG. 2A and FIG. 2B, typically, the liquid crystal molecule 328 over the pixel electrode 308 is in splay state and the liquid crystal molecule 326 over the other region is in bend state. Then, a high voltage is applied between the conductor electrode 324 and the pixel electrode 308 for a given period at the start of operation of a liquid crystal display device using the OCB cell. At this time, the liquid crystal molecule 326 in bend orientation change the orientation state of the liquid crystal molecule 328 over the pixel electrode 308 from splay orientation to bend orientation. However, a part of the liquid crystal molecule 328 over pixel electrode may be unsuccessfully transformed and remain in bend orientation, which reduces the display quality of the LCD. In addition, the two orientation states required in this method increase the manufacturing cost. Moreover, it is difficult to maintain the high angle of inclination of a bend orientation state liquid crystal molecule. Although this allows the liquid crystal display device to have a desired wide view-field angle characteristic, the image quality required for it cannot be obtained easily. Further, the above measure is not practical.
FIG. 2C shows another orientation state in accordance with the conventional method. The liquid crystal molecule 330 in the whole pixel is in splay state. In accordance with this method, a high voltage is applied between the conductor electrode 324 and the pixel electrode 308 for a given period at the start of operation of a liquid crystal display device using the OCB cell to transform the liquid crystal molecule 330 from splay state to bend state. This fixed start time usually takes more than several tens of seconds. The liquid crystal molecule 330 returns to splay state when the LCDs is turned off. However, part of the liquid crystal molecule 330, such as the liquid crystal molecule between the video data line 304 and the pixel electrode 308, is applied to the high voltage in this mode, which cause two liquid crystal molecule states when the LCDs is turned on. Yet another problem is that even if the liquid crystal molecule 330 is transformed from splay orientation to bend orientation at the start of operation, the OCB cell may return to splay orientation during operation. The LCD must be restarted for display to return to normal.
On the other hand, recent battery-driven systems such as notebook-type personal computers equipped with a TFT color liquid crystal display device are increasingly required to be of a power-saving type. To conserve power, such a liquid crystal display device has a driving mode stop function to turn off a display thereof. Once the LCD is turned off, an OCB cell returns to splay orientation from bend orientation. A period of time is needed to restore the bend orientation state; thus the display cannot be turned on instantaneously.